It is not permissible to sell gold for money unless the full price is paid in the same sitting

It is not permissible to sell gold for money unless the gold
and the money are handed over in the same sitting. This is what has been
stated by the fuqaha’. This is called taqaabud (a reciprocal taking
possession of commodity and its monetary equivalent by buyer and seller
respectively), in which the buyer takes the gold, and the seller takes the
money (in the same sitting). It is not permissible to sell gold if there is
no taqaabud. See question no.
2239. 

What you have to do is to explain this to those who buy from
you, and what a Muslim has to do is to listen and obey whatever Allaah and
His Messenger (peace and blessings of Allaah be upon him) have
commanded. You are not doing this because you do not trust them, for
example, rather you are doing it in order to follow the sharee’ah. You
should explain this in a kind and gentle manner. 

Shaykh Muhammad ibn Saalih ibn ‘Uthaymeen (may Allaah have
mercy on him) was asked: 

What is the ruling on giving gold before receiving the money
for it, if it is given to a relative because of the fear of severing the
ties of kinship and I am aware that he will pay for it even if that is after
some time? 

He replied: 

You should be aware of the general principle that it is not
permissible to sell gold for money at all unless the price is paid in full
immediately. It makes no difference if the customer is a relative or a
stranger, because the religion of Allaah does not favour one at the expense
of the other. If your relative gets angry with you for obeying Allaah, then
let him get angry, for he is the wrongdoer and sinner who wants you to
disobey Allaah, and in fact you have honoured him by preventing him from
entering into a kind of transaction with you that is haraam. If he gets
angry or cuts off ties with you for this reason, then he is the sinner and
there is no sin on you. 

Fiqh wa Fatwa al-Buyoo’, compiled by Ashraf ‘Abd al-Maqsood, p. 389.