Sales and Purchase Order - Minimun Quantity

Hello,

I need to resolve that kind of situations

1) If i want to buy the Item A to my Supplier S1 the minimun quantity of items is 500, so i could make an order with 600 but not with 450.

2) If i want to sell the Item B to any Client, the minimum quantity of items is 50, so i could sell 51 but not 49.

3) If i want to sell the Item B to any Client, if have to buy from 10 to 10, so i could sell 80 but not 85.

4) If i want to buy the Item C to the Supplier S2, if have to buy from 5 to 5, so i could buy 80 but not 83.