It is necessary for partnership that the percentage is fixed out of total profit i.e. as much profit is availed; a certain percentage of it is fixed. It is not lawful to take a fixed profit as it shall invalidate the partnership. If you want to invest with your friend then you should fix the percentage out of total profit. In case of loss each partner should bear the loss as per the capital amount and in case of taking fixed percentage from the business usually both these two points are not maintained.
Allah knows Best!
Darul Uloom Deoband