Does an agreement to convert a due amount into advance rental payments covering a long period, along with the issuance of premature rental receipts for that period, fall under the category of Riba (usury) or sin in Islamic Sharia?

A loan that yields benefit to the lender is usurious (riba), based on the saying: "Every loan that draws a benefit is riba."

If the contract between you and the landlord is for a fixed term (وجيبة - determined period), and you hasten payments to him in exchange for being exempted from 18 months' rent, then this is usury (riba).

However, if the contract is month-to-month (مشاهرة - unspecified period), or a fixed-term contract that has expired, and the landlord desires the sum in exchange for exempting you from 18 months' rent, then this contract is valid.

In summary: If the initial contract was for a fixed term (وجيبة), then the new contract is not permissible. If it was month-to-month (مشاهرة) or a fixed-term contract that has expired, then the contract is valid.