What is the ruling on a contract signed by an employee under duress after its term was amended without his knowledge or consent, and is he entitled to complete 15 years and claim retirement, and what is the ruling on rescinding this contract with a medical report or otherwise?

Contracts must be fulfilled, as Allah the Almighty says: (O YOU WHO HAVE BELIEVED, FULFILL [ALL] CONTRACTS) Al-Ma'idah/1, and as the Prophet (peace be upon him) said: (Muslims are bound by their conditions). What was mentioned regarding the contract with the institution could mean one of two things:

1. The contract was made for a period of fifteen years, after which partial retirement could be claimed. In this case, the institution is obligated to fulfill that, and it is not permissible for them to extend the period.

2. The contract was made for a period of seven years, then for four years, and the custom was for the third contract to be four years, but the institution increased the period. There is no harm for the institution in amending its system and changing its contracts, because they are independent contracts. You have the choice between entering into the third contract or not. If you accept the contract, it is not permissible for you to resort to trickery to terminate it, and custom does not override stipulated contracts.