All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger. We ask Allaah to exalt his mention as well as that of his family and all his companions.
If you wish to sell a portion of your share to your business partner, you may do so according to the price on which you both agree. If you want to sell it according to the present price in the market then you may do so, and if you wish to sell it to him according to the price with which you bought it, you may also do so. However, you are not obliged to sell it to him with the same price you purchased it. This is a trade contract on which both parties agree like all other contracts. If you want to sell it according to the present price in the market and the value of the house in the market is 150,000, then he has to pay you 37,500 i.e. a quarter of the value of the house, and so he will possess half of the house and you will possess the other half.
Allaah Knows best.