可以这么说：

Dear manager,

Thank you so much for discussing my work situation with A and B.  Four days WFH in a week, indeed, is a good offer.  I wish this offer have had provided before I decided to move on.  I have already accepted the offer from XXX, with regrt, I would like to resign my position in XXX. My last day is XXX.

Sincerely Yours,

IIyyy

这件事儿你已经赢了，就宽宏大量一点儿，给你的管理和A部门的领导留个好印象。山不转水转，没必要烧桥。