Voldemort used his wand to place some more clauses in the contract. The general idea is that Edward is not to reveal his whereabouts and the fact that he is still alive to anyone--especially Dumbledore.
This has to last until Voldemort himself announces his return to the Wizarding World. And if Edward were to break this contract, he would lose all his magical abilities.
All the additional clauses that Voldemort added were nothing but a smokescreen for his real purpose.
His purpose was to create a loophole in the contract. The loophole was that given the fact that he is technically not a living person or thing, this contract is of no use to him. Instead, the body he is occupying will be responsible for paying the cost for him.
So, if Voldemort signed the contract, Edward would receive all the knowledge and experience of Quirrell, not of Tom Riddle. After modifying the contract to his liking, Voldemort did not immediately sign it as he did not want to alert Edward. Instead, he sent it back for him to check it.
After Edward received the modified contract, he took his time to check it, and he instantly saw the loophole in the contract. However, he did not say anything except secretly sneering.
This contract is one of the greatest accomplishments of Edward in both magic and alchemy. It contains so much knowledge.
First, it contains the Unbreakable Vow and Blood Pact, making it impossible to break. It contains Soul Magic that Edward researched by himself, Memory Magic that he studied from Obliviate, Occlumency and Legilimency, and his research of the Pensieve.
And during his five years voyage, Edward acquired new knowledge that allowed him to further increase the power of this contract.
One of the many books that Edward stole from the Secret Vault of the Vatican talked about demonology. In one book referred to as [The Lesser Key of Solomon], Edward learned how to summon demons, how to control them through contracts, and how to use schemes when signing contracts.
For example, all the beautiful designs on the corner of the parchment were a demonic language that allows Edward to place countless hidden clauses on the contract.
So, he did not care about the little trick that Voldemort thought that he made in the contract. Compared to the ones that he made, it was nothing.
So, after pretending to review the contract for a long time, he used his wand to sign the contract. Then, he sent it back to the Dark Lord, who also signed the contract after making sure that there was nothing wrong with it.
Unfortunately, he soon regretted this decision. As soon as he finished signing the contract, Voldemort felt countless of his memories were removed from his soul and rushed towards Edwards in the form of white strings.
The memories involved all his knowledge, understanding, experience, or skill of magic. Every time that Voldemort used a spell, the experience he felt, the thoughts he had about the spell, the feeling he had. All of them were turned into memory strings and headed towards Edward.
As a master of Occlumency and Legilimency, Voldemort tried hard to prevent his memories from escaping. Especially the ones about his Horcruxes, but it was to no avail. He felt a powerful will forcibly snatch these memories from his very soul.
Additionally, Voldemort felt a deep threat of death as he continued to resist the magical powers of the contract. He has a feeling that he would truly die if he resists, and even his Horcruxes would not be able to save him.
After a few minutes, countless white strings of memories were all around Edward. He took out an alchemy item that looked similar to Dumbledore's Deluminator. With a wave of it, all the memories were perfectly stored inside, and Edward has a happy and satisfied smile on his face.
As for the Dark Lord, he wanted nothing but to immediately kill Edward. However, he knew that he was in no shape to accomplish such a task. However, Voldemort has placed Edward as the second person that he must get rid of after Harry Potter once he is resurrected.
"Edward Bones, do not forget that you are also under contract, so fulfill your end of the bargain," said Voldemort in a hoarse, almost hissing sound.
"That's true," replied Edward. Then, he sent another parchment to the Dark Lord before Apparating long before he even read it.
As for Voldemort, he started to roar out loud after reading a few words from the parchment that said: "Mirror of Erised."
