What is the ruling on a penalty clause for delayed payment of house purchase installments, knowing that a large portion of the amount was paid before signing the contract, and the clause stipulates a 10% late fee if the installment payment is delayed for 30 days?

Stipulating a fine for late payment of an installment is a forbidden and void condition, and it is the usury of the Ignorance (pre-Islamic era). Therefore, it is impermissible to fulfill it. However, since you have signed the contract and cannot withdraw from it, and you have paid a large sum of the price, there is no harm in proceeding with the contract and paying the remaining installments, while being careful not to delay payment to avoid this fine.