How can I help a homeowner in pre-foreclosure if he wants money?

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[I need help. Please read.]

There is a homeowner in pre-foreclosure. He has 40k equity and the house is worth 120k he is 5k behind on payments. If I buy it for 100k, he will have 20k equity left. How can I give the seller his 20k in equity? Can I make him payments or can I tell him I will pay him in 3years? When you take over someone's else payments do give them some money or do you just take his payments? What options do I have?