I guess you have Quote ID in both Quote table and Sales Instruciton table. so the two table can have relationship based on this Quote ID.
And the relationship will set up to create a record for the Sales Instruction side whenever you create a Quote record.
the relationship will be one to one.
That way you don't need to validate anything. you can have all the related fields from both tables showing on one layout.
Give it a try.
Or use a script to check if there is record existed in Sales Instruction when you click on the button on the Quote table. This way it also needs a relationship created
between Quote and Sales Instruction based on Quote ID / #.
Hope it helps.
I am not able to find out that script for validation. can u give me an smiple script.