Contract Law Problem Question Help PLEASE!!!

Hi,

I am so confused with regards to Consideration in a problem question.

R promised his friends S, T and U to sponsor their participation in a Lands End to John O’Groats bicycle ride in aid of the Contract Lecturers’ Benevolent Fund. S lacked the funds to buy a suitable bicycle so R lent him £600 on terms that S would repay him at the rate of £100 per month after the sponsored ride had been completed. Six months later S was three months in arrears with his payments and told R that he had lost his job and could only pay £50 per month for the time being. R agreed to accept £50 per month but after S had made two payments R heard that S had won a large sum in a lottery. R wished to claim the balance of the sum still owing immediately.
Advise R.

What part of consideration is this referring to? Is it referring to an existing contractual duty or a discharge of existing financial obligations? If anyone can help I'd be most grateful, thanks!