Transact SQL :: INSERT Specific Data To Other Table

I am still learning T-SQL .Lets consider the table below, ID 1-3 shows our purchase transactions from various Vendors and ID 4-6 shows our payments to them

Table 1 - VendorTransactions

ID     PARTY    AMOUNT   VOUCHER
---------------------------------------
1       A               5000              Purchase
2       B               3000              Purchase
3       C               2000              Purchase

4       A               3000              Payment
5       B               1000              Payment
6       C               2000              Payment
7       A               1000              Payment


Now we have a blank table Table 2 - Liabilities

ID     PARTY    AMOUNT 

I want that SQL should look for each individual party from Table 1 and Calculate TOTAL PURCHASE and TOTAL PAYMENTS and then deduct TOTAL PAYMENTS from TOTAL PURCHASE so we get the remaining balance due for each party and then add the DIFFERENCE AMOUNT alongwith PARTY to the TABLE 2 so I can get the desired result like below

ID     PARTY    AMOUNT
-------------------------
1       A               1000              
2       B               2000              
3       C               0