how to solve it in laravel and mysql ? help me to solve it

Fetching data from table transaction and inserting into the same table, The example I have balance $50 and I purchased $5 then I have remains $45, but when multiple users purchased at the same time, balance is $50, three users buy $5,$10,$5 respectively. I get the remaining balance as $45 how??? enter image description here

via Chebli Mohamed