Early settlement discount

Q: We have customers who buy on credit. Is it permissible for us to tell them that we will give them a discount if they pay before the due date? For example, the customer purchases goods worth R100 000 on a three month credit basis. The customer is told, “If you pay within one month, you will receive a 10% discount”. Is this permissible? 

Stickiness remaining after removing a plaster

Q: Due to a small tiny cut on my left knuckle middle finger I put a plaster on it, I removed the plaster afterwards and washed my hands but realised later after that there was still some residue of the plaster stickiness/glue left so if you rubbed it off it would form like a sticky rubber effect. Because I may have washed impure hands afterwards whilst the stickiness was there and not realising and then wiped them with a towel, I am concerned impurity may have been spread or would be pure as I initially did wash my hands but did not realise the stickiness was still there. I am a waswas sufferer and I am getting better everyday with the help of the almighty therefore I am always very concerned on purity as I always think I am spreading impurity everywhere.