Unify stock for all atributes in one product

[Zeryk]

PS1.6.1.12

 

I sell loose tea. Each product ( tea ) has attributes 50 / 100 / 250  / 500 / 1000g .

The reason for this separation is being for different prices ( more u buy cheaper it gets per gram ) . 

The problem is if somebody buys , lets say, 100g - 1time ...it will change a stock number in 100g attribute but not the others. Which leads to the problem that I have to update stock for other attributes manually. 

( because if I sell 100g  to 10 people , the attribute 1000g stock  will not change. )

 

Is there any solution , plugin for PS to sort this out?