Is it possible to fulfil the Mitzvah of writing a Sefer Torah by writing a letter in a not yet completed Torah.
According to many poskim a person fulfills the mitzva of writing a sefer torah when he not merely sponsors but buys a part of a sefer torah. Even though he is not the one who actually wrote the sefer torah, however when he buys a part, he is making the sofer his messenger to write his part of the sefer torah. It is skeptical if one fulfills the mitzva by merely writing a letter in the sefer torah if he doesn’t own it.
Shut Chesed L’Avrohom 1 Y:D 84, Piskei Teshuva Y:D 270-1, Bais Halevi 30, Igros Moshe Y:D 1 163, Teshuvos Vhanhagos 1-624, 2-519, Derech Sicha pg. 587.