What is the ruling on selling by installments, and what is the legitimate solution to address buyers' default on paying some installments?

Firstly: It is permissible for the deferred price to be higher than the cash price. Mentioning the price in cash and in installments is allowed, and the sale is valid if the two parties definitively agree on either cash or deferment. It is not valid if there is hesitation.

Secondly: It is not permissible to stipulate in the deferred sale contract additional installment charges separate from the cash price.

Thirdly: It is not permissible to obligate a late buyer to pay an increase on the debt, as that constitutes usury (riba).

Fourthly: It is forbidden for a solvent debtor to procrastinate, and it is not permissible to stipulate compensation for delay.

Fifthly: It is permissible for the seller in a deferred sale to stip stipulate the acceleration of installments before their due dates if the debtor delays, provided the debtor agrees to this.

Sixthly: The seller has no right to retain ownership of the sold item after the sale. However, it is permissible for the seller to stipulate pledging the sold item as collateral to guarantee their right.

Seventhly: A reduction from a deferred debt for prompt payment is permissible if it was not by prior agreement and the relationship is bilateral.

Eighthly: It is permissible for the two debtors to agree on the acceleration of all remaining installments if the debtor refrains from paying any installment, unless the debtor is in financial hardship.

Ninthly: If the debt is considered due upon the death of the debtor, or their bankruptcy, or their procrastination, it is permissible to reduce it for prompt payment by mutual consent.

Tenthly: The criterion for financial hardship that necessitates a grace period is that the debtor does not possess property exceeding their basic needs that would suffice to pay off their debt.