All perfect praise be to Allaah, the Lord of the worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His Slave and Messenger.
It is alright to go on with this deal, but you should advise the contractor and tell him that what he will do is plain Riba and point out to him that Riba is prohibited as it is considered fighting Allaah The Almighty and it brings about His curse. You should mention to him the verses and Hadeeths (narrations) that indicate this. Allaah The Almighty Says (what means): {Allaah destroys interest and gives increase for charities.} [Quran 2:276]
The Prophet, sallallaahu ‘alayhi wa sallam, cursed the one who took or gave Riba. [Al-Bukhari] It was narrated that the Prophet, sallallaahu ‘alayhi wa sallam, cursed the consumer of Riba and its payer, and the one who records it, and the two witnesses, and he said: “They are all equal (in sin)." [Muslim]
Allaah Knows best.