That opens the door to the 24-year-old's possible exit from Parc de Princes during this summer's transfer window, the source added, confirming a report in French sports daily L'Equipe.
Mbappe's confirmation by letter comes a little over a year since he snubbed Real Madrid by signing a surprise extension to his contract to remain at the French champions.
PSG will not entertain allowing their prize asset to leave for free when his existing deal runs out.
This raises the possibility of the World Cup runner-up switching clubs if talks with the PSG hierarchy break down between now and the end of July.
Mbappe's letter comes hot on the heels of Lionel Messi's departure to Inter Miami.
It came as little surprise as reports had suggested he would not exercise an option to stay until 2025.
At an awards ceremony last month where he was voted the Ligue 1 player of the season for the fourth successive time after helping Paris Saint-Germain win a record 11th French title, he was asked about his future.
"I will be here again next season" was his reply.
