He gave his car to someone to sell it and buy a newer model for him. Is this riba?

What you and your relative
did is riba which is haraam, because what the transaction really meant was
that you lent the car to him on the basis that he would give you back
something better than it, and the scholars are agreed that any additional
amount of benefit that the lender takes from the borrower in return for the
loan is riba. 

Ibn Qudaamah (may Allaah
have mercy on him) said:

“Every loan in which it is
stipulated that something extra must be paid back is haraam, with no
difference of scholarly opinion. Ibn al-Mundhir said: [the scholars] are
unanimously agreed that if the lender stipulates that the borrower must pay
extra or give him a gift, and he lends him money on that basis, then
accepting the extra payment is riba. It was narrated from Ubayy ibn Ka’b,
Ibn ‘Abbaas and Ibn Mas’ood that they forbade every loan that brings
benefits…”

End quote from al-Mughni
(4/211). 

So you have to repent and
seek forgiveness for this action. 

With regard to what your
relative has to do after selling the car for a low amount: 

It seems that he should pay
the value that it was worth at the time when he took it from you, not the
price that he sold it for, because the contract between you was not valid in
the first place, and what he did after that of selling it for a low price is
irrelevant, unless you let him off, waive your rights and accept the price
for which he sold it. 

It may be that this
transaction was a sale, not a loan, in the sense that you sold him one car
for another. If that is the case, then the sale was also haraam and invalid,
because the price was unknown, namely the car that you will get, because you
did not agree on any of its features except that it should be newer than
your car, and this description is not sufficient and does not alter the fact
that it is unknown. 

Whatever the case, whether
the transaction was a loan or a sale, it is haraam, and your relative must
return to you the price of the car at the time when he took it from you. 

And Allaah knows best.