Sandbox account missing payment methods when approving created payment - php

Question

I am trying to implement PayPal REST API.
When I create PayPal Payment using request from my server, the payment is created successfully, and payment id is passed to PayPal.
Next, the user must approve payment.
PayPal window is opened in user's broswer, and user is prompted to login or pay with credit card.
The problem is:
When I login with my sandbox customer account, PayPal is not showing any available attached credits cards or paypal wallet balance, it only prompts to add new credit card to sandbox account.
If I login with this sandbox account in new a separate browser window, I can see that it has a wallet balance and attached credit card.
If I create a Payment with client-side JS request instead of server, the issue is not reproduced and PayPal is showing both wallet balance and credit card.
So, I assume the issue lies in the payment's format. The server Payment's format is:
{
"intent": "sale",
"payer": {
"payment_method": "paypal"
},
"transactions": [
{
"amount": {
"total": "10.00",
"currency": "USD"
},
"description": "silver membership",
"custom": "account_id:1"
}
],
"redirect_urls": {
"return_url": "https://example.com/return",
"cancel_url": "https://example.com/cancel"
}
}
The format of payment that created with client-side JS is way more simple, I took it from one of examples in developer's manuals. If I use the same simple format in server request, it would not allow me to create payment (I think client-side checkout.js just add missing required fields silently).
payment: function (data, actions) {
return actions.payment.create({
transactions: [{
amount: {
total: '10.00',
currency: 'USD'
}
}]
});
},
Would be very grateful for any hints on this topic.