Salih bin Kaysan (may Allah have mercy on him) said that he (peace and blessings be upon him) was asked: A man owes make-up fasts and he fasts one or two days separately; will that suffice him? So he (peace and blessings be upon him) said: "If someone owes a debt and keeps paying one or two dirhams, his debt gets paid. Would he not in this way be freed of his obligation?" They said: Yes. So he (peace and blessings be upon him) said: "In the same way they will be fulfilled on his behalf."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصيام / 8242