I see, thanks for the confirmation.
If this is the case, as explained in my first reply on this post, stores that aren’t using Shopify Payments won’t be able to use the local currency feature that Shopify Markets offers. There isn’t any other workaround for this, using Shopify Payments is a requirement in order for stores to enable this feature.
Out of curiosity, which payment provider is your client using on their store? Is there any particular reason why they’ve chosen that payment gateway, and not Shopify Payments?
For context, Shopify Payments is available in Singapore, so if your client is eligible and meets the gateway’s requirements and Terms of Service, you can look into setting up Shopify Payments on the client’s store to take advantage of the many features and benefits of our native payment gateway.