Moneyman
You got it!
It makes no difference to you what the seller is selling the property for and what if any profit he made on it. That is between him and the tax authorities. What is important to you (for repatriation purposes) is the amount of money you 'officially' pay for the property, ie. what is stated on the Compromise de Vente, not what you 'actually' paid for it. I assume in your example that on paper at least, you would be paying officially only £50,000 for the property. In this example, try to get the seller to sell you the property for £70 'officially' and then do a seperate contract with him where he returns the £20,000 to you. Now you are only out £20,000 for repatriation purposes, although he may not want to do this because he will be paying tax on an extra £20,000.
I am not sure on what condtions the seller is selling you the property, ie. is all the money going to be paid to him in Morocco or does he want some of it in the UK, if so you are right, this amount is not taken into consideration as payment for the property and it will be sold to you 'officially' at the lower price. What is important for the buyer is to show more money on the Compromise de Vente and for the seller, it is to show less money. Negotiations often fail becasue of this particular point.
What is important for you to remember is that when you come to sell, you do the exact opposite in trying to get the Compromis de Vente to show a lower amount than you actually receive.