If one sells an item at Rs.10 against cash payment, can he demand a higher price from a a buyer who wants to buy on 30 days credit. IN other words:

If an item is sold for Rs. 10 against cash payment, can the seller demand Rs, 11 for the same item from a buyer who wants to buy against 30 days credit. This question needs clarification in the light of Riba .

Praise be to
Allaah.

It is permissible to sell an item on credit for a
higher price than if it is paid for immediately, whether the payment
is to be made in installments or in one payment at the later date. But
this is conditional upon the two parties not separating until they have
agreed how the transaction is to be done, whether it is to be paid immediately
or deferred. In this case the additional amount is not riba, and there
is nothing in sharee’ah to specify the amount of the extra payment in
the case of deferred payment. But the Messenger of Allaah

(peace and blessings of Allaah be upon him) encouraged tolerance in
matters of selling, buying, paying off debts and asking others to pay
debts. 

Fataawa al-Lajnah al-Daa’imah 

See Fataawa Islaamiyyah,
part 2, p. 335

But if
he sells it for ten, for example, then the purchaser subsequently wants
to delay payment, it is not permissible for the vendor to add anything
to that. If he adds something in return for extending the time period,
this is riba and is haraam and a major sin. We ask Allaah to
keep us safe and sound. 

See
also the answer to question no. 1231.