I see her predicament a lot as a pet groomer. And the combination of extra weight, the dandruff and the mats go hand in hand.
The mats are hair that was meant to shed but didn't. It WILL comb out. Get a shedding comb. It is a metal comb with alternating long and short teeth. Then use a "picking" motion instead of combing with long strokes. (keep this saying in mind....inch by inch it is a cinch, yard by yard it will be hard). You can only do a very small section at a time.
You can also usually work those mats loose with your fingers. But whatever you do DO NOT USE SCISSORS You will cut her delicate skin and then you will have an even more expensive vet bill.
If you cannot get them out go to a groomer. I know you claim to not have money, but this is VERY bad for her skin and health. The mats prevent air from reaching the skin, and the mats themselves pull and hurt (probably even more than the combing of them out).
After they are out you need to brush her regularly. This will catch the shedding hair before it can mat. She may also need a change in food as this is the most common cause for the vicious cycle of weight gain, dandruff and mats.