updating transactions with preaggregated data - sql

Question

I have a transactions table. I need to update the table with a pre-aggregated value from another table and then roll these down to varying levels of granularity.
However, the final output is incorrect. Hoping someone here can help me figure out how to go about this.
Table A:
TransID BankName Location Region SaleType MonthlyPayment Weight
1 BOA Boston East F 3000 3
2 Mellon Pittsburgh East C 1000 3
3 BOA Boston East C 2000 2
4 BOA Boston East 1000 2
Table B
BanKname Location Region Sales
BOA Boston East 500
Mellon Pittsburgh East 1000
Desired Output structure
BankName Location Region SaleType AvgSales AvgMonthlyPayment
Issue is that when updating and doing the weighted average, each of the Boston transactions is getting the 500 sales. When added, total sales should be 1500 but is now 2500.
If I update Table A with Sales value from Table B, sales gets repeated for each saletype - so it throws off the final average sales.
update is this: (Added new column sales in A)
update a
set a.sales = b.sales
from tableA a join tableB b on a.bankname=b.bankname and
a.location=b.location and a.region = b.region
weighted average from A is calculated like this:
select bankname,location,region,saletype,
sum(case when sales is not null then sales*weight else 0
end)/sum(weight) as avgsales
, sum(case when monthlypayment is not null then monthlypayment*weight
else 0 end)/sum(weight) as avgmonthlyp
from tableA
group by bankname,location,region,saletype
For each saletype, the sales is updated with value thus increasing the final value by that fold.
How can I update sales so that BOA only gets 500 and Mellon only gets 1000 and the total # sales is at 1500?