New info concerning tithing

This is something I learned this morning in Bible Study. We were discussing how people in debt are suppose to tithe. Many times we have heard it taught that everyone must tithe on the gross. The point came up about people who can't tithe on the gross since they will not have enough money to pay their creditors. Well this is what we learned. Let's say there are two people who make $3,000 a month. Person A: Is a married woman with 3 kids. After taxes she is left with $2,277. Her obligations are to pay half the mortgage ($500.00 and to pay the $59.00 cable bill, and other things which equals up to an additional $250.00. After everything is said and done she is left with $1468). Her husband takes care of the other expenses since he brings home $4,000 a month. Her tithe will be off of the gross ($300.00). Person B: Is a single mother of 3 kids. She also brings home $3000.00 and lets say she is left with $2,277 after taxes also. Let's say her mortgage is $700 (she also has the electric, water, cable, telephone bill which all equals to $350.00). Childcare is an additional $400.00 a month. So she is already down to $827 and we haven't even got to her car payment which takes off an additional $200.00 leaving her with $627. Since she is a single mom, she needs money for gas, groceries, and other things that may come up. So her tithe can actually be off the $627.00 which will be $62.00. Because that is all she has left once she has fulfilled her financial obligations (her vows). This is because God has a commandment concerning fulfillment of vows and if you have financial obligations that will maintain your household you have to fulfill them and cannot delay the payment of them (Deut. 23:22). And making a vow (promise) before man is like making one to God Himself. So God doesn't want a tithe that includes part of her mortgage, or that will cause her children to be without childcare. Not only will you be not fulfilling your vows but you will also be stealing (from the people you owe money to, the childcare workers, mortgage company, etc). This goes for anyone who is in debt. Because what is left in your hand after you have paid your debtors truly belongs to you. However, this is only a temporary situation which will enable a person to increase their level of tithing. I am not here to stir controversy because there are different teachings concerning tithing.