Taking money that does not belong to them

On the 24th January my wife paid Lake Smit and Partners for a scan. The claim was submitted to our medical aid (Moto Health) for a refund. They incorrectly refunded Lake Smit and Partners. I phoned them and asked them to transfer the monies to our account and they refused stating that the over payment was being kept for a 2nd scan. The 2nd scan submitted by their own personnel at Pinetown contained the incorrect code and was thus declined by the medical aid. I also told them that they could not hold back or keep monies due to us and incorrectly paid to them as a surety. After several frustrating calls they have conceded that what they did was wrong but will transfer our monies, paid to them on the 29th January, on Monday 29th February almost 1 month after taking our money and earning interest on this.