unlawfull enrichement

Company X supplies consumables to company Y, for the past five years they have been fine up untill the past three months, they have had issues with some of the consumables,which has been replaced immediatly, how ever the payments due for the past two months have been held back, and the purchasing manager is accusing company X of supplying inferrior quality goods, and will hold payments due untill a 50% disscount is been given on products involved for the past six months.which we are not agreeable to, we have sent a letter of demand giving them a 10% disscount on the products involved for the past two months,this has been replied by saying that we have to accept we supplied inferrior quality goods and a 50% disscount should be given as requested before.is this legal under business law, i am an employee of company X
Thank you