The basic position is that a solicitor should release funds to their client as soon as possible but there could be any number of reasons for not doing so.  For example, the proceeds of sale might belong to the estate of a deceased seller, and the estate is yet to be finalised and distributed.  It might be that there is a dispute between co-owners about the funds and who should get what, in which case the solicitor might have received conflicting instructions as to which of their clients receive what proportion of the proceeds of sale.  It could be that a retention of some sort was agreed for something.  This is not uncommon in leasehold property where service and other charges by the landlord are not certain at the point of completion, so the parties might agree that a certain sum be held back in one of the solicitor’s account until that figure is clear, at which point it can be sent to the seller after deduction of any such charges.