If the seller is in possession and the due diligence (including that of a surveyor) does not bring up any encroachment issues, you could go ahead and buy and retain a portion of the purchase price until the deslinde is done.
Also, a new resolution by the Supreme Court, just published, states that "undeslinded" properties dating from before April 2, 2007, can still be transferred, for just one time, provided that the owner is selling the entire property and not a piece of it!
For example, if you have a "Carta Constancia" (an undeslinded property) from before April 2, 2007, for 1,000 square meters. The Supreme Court is now saying that you can convey the property to a buyer if the whole 1000 meters are sold. The new owner, however, will not be able to sell it without doing a deslinde.