I will give you #5000 on Monday, how sure are you that the money will come in on Monday? Promises and assumption has caused alot of people there job, imagine leaving your recent job because a friend promise to get you a better job. This may even cause someone his/her life if one base most of his/her lifestyle on promises and assumption.
I think that if a person promises to return your money on due date then may be he is true and surely he will find some way from which he will get money soon. Promises are not fake and once a person promise he will fulfil it but if unfortunately he can't then may be he has some necessitation and we should understand this thing.
And as you are asking a question about a job so promises for getting cash and promises for job are two different thing. You can trust on your relative and friends but not too much to allow him to destroy your whole life. Giving money to another person as a loan is not bad thing but leaving your job for more money just because your friend forces you is not a good thing and it is a type of greed which you will regret in future.