[true_religion]

While a deed of assignment is typically used to transfer existing rights and possessions, there's no real reason it cannot be used to transfer future rights.

Deeds of escrow are often used as promises to transfer goods, or perform a service in the future even if conditions for said execution of the deed are not currently met.

Legally it is defensible. To fight against its validity, you'd need to go another route---like showing that one party had undue influence over another.

In this case, it would be a muddy ground battle, because they have already left their employer so there's no influence, and the language of the deed is plainly understood.