someone promised me to help solve my problem but he did not help me. he promised me again but still intentionally make excuses. or someone borrowed money from me and gave me many dates to repay but intentionally he gave me so many excuses every time. is there any idioms or phrases to explain their intentional dodgy behaviour?
I think the word you're looking for is "deceptive". They are promising to help you with your problem, or pay back your debt, yet their promises don't reflect their intentions.
Use the adjective "habitual" to make clear that behaviour is repeated, e.g. "a habitual liar", a "habitual promise-breaker" (or "habitual promise-maker" with the implication that he makes them but does not keep them) or a "habitual borrower". We most often think of habits in terms of bad habits, so "habitual" sounds rather disapproving although strictly speaking it is neutral.
Another term for a person who borrows repeatedly without paying back the loan is "a bad credit risk". This is quite a recent coinage from the world of business. It could apply to both someone who keeps borrowing without any intention of ever repaying the loan and to someone who, while having no intent to deceive, keeps failing to repay due to mental incapacity, addiction, or habitual (that word again!) over-optimism about their ability to earn money.
In order to specify that the repeated borrowing was done cynically, you could say that the person was a "habitual borrower in bad faith".
To describe the person who always promises to help but never does, a colloquial expression is "he's all talk". A specifically British version of that is "he's all mouth and no trousers". A related American expression is "he talks the talk but he doesn't walk the walk". Those apply to any sort of failure to match deeds to words. To narrow it down to your example, you could say, "he's always willing to help, in theory." People will get the implication that theory and practice differ.
Both the people you mentioned could generally be called "untrustworthy".
: not worthy of trust or belief; an untrustworthy person.