This mod will update every visited product (before being displayed and before being displayed in cart) if its stock differs from the one in the csv file.
V1.0

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For Opencart 2.3.x

Install:
If you don't want to set up your ftp details:
Rename install.xml to qtyupdatefromcsv2x3x.ocmod.xml and upload only this xml file in extensions installer.

Refresh the modifications cash after install.

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For Opencart 3.x

Install:
Upload the whole archive in extensions installer.

Refresh the modifications cash after install.

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For any version add to config.php at the bottom:

define('UPDATE_STOCK_CSV_FILE', 'ftp/test.csv');

Replace ftp/test.csv with the path to your csv.
(ftp is in the same folder as config.php in this example, so it must be created.
You can use '../ftp/test.csv' to place this folder outside your opencart installation).
Please make sure the csv file is not accessible in browser via direct url.

Configure a regular update of the csv file via ftp from your ERP management program with the follosing structure:
ean,stock

Example:

1234567891234,10
1234567891235,0