Is it permissible for the landlord to accept 250 Egyptian Pounds of the agreed-upon monthly rent value in the contract (300 Pounds) and consider 50 Pounds as sadaqah (charity) or a deduction from usurious money owed by him?

It is permissible for the landlord to waive a portion of the rent for the tenant with the intention of charity, and he will be rewarded for that, whether the tenant is poor or rich, for voluntary charity is valid even for the rich.

However, if the questioner intends to deduct usurious money from the rent, he must dispose of it immediately. He is not allowed to delay it or benefit from it, nor should he give it with the intention of charity, but rather with the intention of getting rid of it, because Allah is good and accepts only what is good.