Second amount not being added

Hi,
I have custom script in Wordpress, there's a small part of the script that doesn't work
Very simple task

Pay attention to line 461, the part where the second trade amount is entered
The problem is the script doesn’t save the second amount, examples attached

Here is where you enter the details in the backend: (attached 1-backend.jpg)

And here you will see the second line (“10” in the example above) does not save
This is the example of the backend not saving: (attached 2-backend-missing.jpg)

Here’s an example where it doesn’t save in the frontend: (attacched 3-frontend-missing.jpg)

see the attached php file also