Will these two SQL transactions come to a halt?

Assume I had a buy transaction that went like this:

UPDATE `USER` SET `Money` = `Money - '100';
UPDATE `ITEMS` SET `Item` = `Item` + '1';

Unfortunately, a cron was running at the same moment, paying interest on everyone’s existing balance:

UPDATE `USER` SET `Money` = `Money` + '50';

(either in the form of a transaction or as a single autocommit)
I saw this video in which two or more transactions must wait eternally for each other in order to be finished. Would this result in a deadlock? (Which the dbms may resolve by rolling back one).

To avoid dead locks, one should usually acquire locks in the same order throughout all transactions. I don’t think that your example would run into a deadlock, but since you are updating the whole table it might be necessary to issue a table lock first (maybe the DB detects that on its own).