Is it permissible for the seller to demand the price difference for an item he left as a trust with the buyer for a specified period, after its price has increased?

If your neighbor gives you flour as a loan or places it with you as a deposit, it is not permissible for you to demand an increase beyond what was given to you.

It is permissible to take a known fee for safeguarding a deposit when the contract is made, similar to the fee for rescuing a drowning person or teaching Al-Fatiha.

The Malikis prohibited taking a fee for safeguarding a deposit, but permitted taking a fee for the place where the deposit is kept. However, their relied-upon position makes no distinction between the two types of fees.

As for flour given as a loan, it is not permissible to stipulate an increase for either the lender or the borrower beyond merely returning the loan.