Increase field value based on condition

Increase field value based on condition

So I have a picklist inside of the zoho forms.

In the picklist I have 6 coresponding options. Each option has the same name as different currency fields inside zoho CRM.

It looks like this - zoho forms picklist:
Stopnie + Podstopnie
Konstrukcja
Balustrady
Malowanie na mokro
Malowanie proszkowe
Pozostałe

Besides the picklist there is also one currency field inside of zoho forms named "Wydano".

In zoho CRM each deal has 6 currency fields unique for each deal. So you will find the fields named like so:
Stopnie + Podstopnie
Konstrukcja
Balustrady
Malowanie na mokro
Malowanie proszkowe
Pozostałe

Now depending on what user selects on the pick list, after the form is submitted, I want the "wydano" field from zoho forms to be added to the corresponding field in zoho crm. This way whenever someone adds for instance additional cost to "konstruction", the cost showed in the CRM is increased. Its crutial that the currency field would increase by the amount filled by the user, not replaced.

I asked about it on zoho forms and aparently its not possible directly from the forms as it will always replace values. What I could do is to create temporary fields thst will store the amount I want another fields to be increased. 

Then its the matter of creating a function that depending on choice would increase certain field by certain amount. 

How can I do it? 

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