Is it permissible for the owner to ask for an increase in the price of the house after agreeing to sell it in installments and before its handover, due to rising prices?

The price considered valid is that which is agreed upon between the seller and the buyer at the contracting session. Once the session concludes and they part ways, the contract is finalized. The seller does not have the option to increase the price, nor does the buyer have the option to decrease it, based on the hadith: «When two men engage in a transaction, each of them has the option [to revoke the sale] as long as they have not separated and are still together, or if one of them gives the other the option. If one of them gives the other the option, and they transact based on that, then the sale has become binding. And if they separate after transacting, and neither of them abandons the sale, then the sale has become binding.» And "the sale has become binding" means it has become obligatory and complete, and the seller does not have the right to increase the price, even if the buyer delays payment.