The 22-year-old is set will depart for France at the end of the Currie Cup season.
Bulls high performance manager, Xander Janse van Rensburg, said domestic teams were finding it increasingly difficult to compete with offers from overseas clubs.
“We tried our utmost best to retain the services of Paul. It is, however, very concerning that we are losing players at a younger and younger age to the overseas market,” Janse van Rensburg said.
“In Paul’s case, we are talking about a massive offer to the player and one that is very hard to say no to.
“We certainly cannot blame Paul for accepting such a lucrative offer, but even if we wanted to, we could not match it as this is one of the biggest offers I have seen for a player of his age.”
Willemse plans to return to South Africa in two years’ time to make a bid for the Springbok jersey and could well be back in Bulls colours then, Janse van Rensburg said.
“Paul feels that he is not currently in the mix for Springbok selection with a number of locks ahead of him in the line-up, so he wants to add a different element to his game by playing in the French Top 14. Game time in France will see him improve as a player and we can reap the benefits of that upon his return.”