All perfect praise be to Allah, the Lord of the worlds. I testify that there is none worthy of worship except Allah and that Muhammad, sallallahu ‘alayhi wa sallam, is His lave and Messenger.
It is not obligatory to register the land under B's name because this is considered a mere legal procedure; it is sufficient to make a real purchase of the land for B, and then when he acquires full ownership of it and takes it into his possession, he may sell it to A for the price which they agree upon. A can then register the land in his name directly from the original seller. B could have purchased the land himself from its owner over the phone, and he can also sell it to A over the phone as well after taking it into his possession.
The "Islamic Fiqh Academy Magazine", issue no. 6, vol. 2, reads, "Contracts conducted over the phone and the like are valid as long as the transaction does not entail the immediate receipt of the purchased without problematic aspects..."
In any case, if B has sold the land to A after buying it and having assumed full ownership of it, then there is no harm in this transaction and the land has become the property of A, and A then has the right to register it under his name directly before it is even registered under B's name.
As for the extra payment (other than the one included in the agreed-upon price) which A wants to pay to B in return for delaying the payment, it is impermissible, even if he cunningly paid it as help in buying a car or the like. This is because an increased amount charged on the principal amount of a debt in return for delaying the repayment is considered Riba (interest). However, there is no harm if he really wants to help B and give him an extra amount at the time of payment without prior agreement or collusion in this regard. Abu Hurayrah, may Allah be pleased with him, narrated that a man came to the Prophet, sallallahu ‘alayhi wa sallam, demanding his debts, and he behaved rudely. The Companions were about to intervene, but the Messenger of Allah, sallallahu ‘alayhi wa sallam, said (to them), "Leave him, for the creditor (i.e. owner of a right) has the right to speak." The Messenger of Allah, sallallahu ‘alayhi wa sallam, then said, "Give him a camel of the same age as that of his." The people said, "O Messenger of Allah! There is only a camel that is older than his." He said, "Give (it to) him, for the best amongst you is he who pays the rights of others handsomely." [Al-Bukhaari and Muslim]
Allah knows best.