Correct spelling, explanation: the word transfer comes from Latin transferre. The word underwent a few changes over centuries and became adapted to English grammar – in most cases of words of Latin origins, the suffixes such as -ere, -are, or -re were rejected, to make the word as English as possible and this is what happened here, making the word end with only one r.

Definition of transfer:
noun
1. an act of transporting/moving something from one place to another
The biggest challenge was the transfer of all our goods to the buyer.
The transfer of files from a USB to a hard drive is almost done.
verb
1. transporting or moving a thing from one place to another
I decided to transfer all of my money to a savings account
My dad wanted to transfer all of his knowledge to his children.
2. to change school/workplace
Because of his bad behavior, the school decided to transfer him to another facility.

Incorrect spelling, explanation: the word does, in fact, come from Latin tansferre with double r, but it’s important to remember that the word changed a lot to fit into English, and, what is more, if you think about it, there are very few words in English that end in rr. Double r is also unnecessary due to pronunciation and all of that leads us to the correct form: transfer with one r.

Incorrect spelling

Incorrect spelling

Incorrect spelling