USE TestDB
DROP PROCEDURE  IF EXISTS dbo.move_money;
GO
CREATE PROCEDURE dbo.move_money @bill_from_id int, @bill_to_id int, @sum float
AS
declare @money_from float,@money_to float;

select @money_from = bills.money from dbo.bills where bills.id = @bill_from_id;
select @money_to = bills.money from dbo.bills where bills.id = @bill_to_id;

if @money_from  is not null and @money_to is not null
begin
    if @money_from > @sum
    begin
    update dbo.bills set bills.money = bills.money - @sum where id = @bill_from_id;
    update dbo.bills set bills.money = bills.money + @sum where id = @bill_to_id;
    select 'Success! ';
    end
end
else
select 'wrong @bill_from_id and @bill_to_id';

SELECT *, @money_from, @money_to FROM dbo.bills;
GO