string credit = 10000;

text credit_txt
{
	pos_x = 774;
	pos_y = 305;
	layer = 2;
	font lcars2_font;
	string = credit;
	layer = 1;
}

var credits = 10000;

function tile_event() 
{
	if (bmapno ==1) 
	{
	credits = - 2000;
	ent_create(ship_b1, my.x, ship_function);  // create / place ship at my.x?
	}
	if (bmapno ==2)
	{
	ent_create(ship_b2, my.x, ship_function);  // create / place ship at my.x?
	}
	if (bmapno ==3)
	{
	ent_create(ship_b3, my.x, ship_function);  // create / place ship at my.x?
	}
}

STRING credits_s[256];  // 256 may be excessive
var credits_n = 10000;
TEXT credits_t
{
	pos_x = 774;
	pos_y = 305;
	layer = 2;
	font lcars2_font;
	string = credits_s
	layer = 1;
}
// convenience functions
function creditsf_update() {
	str_for_num(credits_s, credits_n);
}
function creditsf_add(_n) {
	//if (credits_n < 0) {
		//credits_n = max(credits_n + _n, 0);	// -credits not allowed
	//} else {
		credits_n += _n;
	//}
	creditsf_update();
}
function creditsf_set(_n) {
	credits_n = _n;
	creditsf_update();
}

function tile_event() 
{
	if (bmapno ==1) 
	{
	//credits = - 2000;
	creditsf_set(-2000);
	ent_create(ship_b1, my.x, ship_function);  // create / place ship at my.x?
	}
	if (bmapno ==2)
	{
	ent_create(ship_b2, my.x, ship_function);  // create / place ship at my.x?
	}
	if (bmapno ==3)
	{
	ent_create(ship_b3, my.x, ship_function);  // create / place ship at my.x?
	}
}

// maybe try?
function creditsf_add(_n) {
	if (_n < 0) {
		if ((credits_n + _n) < 0) {
			return(0);
		}
	} 
	credits_n += _n;
	creditsf_update();
	return(1);
}


// in main function (or use starter)
function main() {
	// level load 
	creditsf_add(10000);
}
//...
	// buy ship code
	if (creditsf_add(-shipCost)) {
		// can afford
		// transaction OK
		// add ship
	} else {
		// come back with more dough captain
	}
	
//...
// put ship back
creditsf_add(shipCost); // refund full cost

string credit = 10000;  // <- can't set string value directly
// must use str functions (look in manual)
STRING credit[256];	// changed
var credits = 10000;  // to display on screen, this value should be copied to STRING credit 
// ...
// credits was modified?
// convert var to STRING
str_for_num(credit, credits); // converts / copies credits (var) to STRING credit

string credit = 100;  // <- error!?

function tile_event() 
{
	if (bmapno ==1) 
	{
	creditsf_add(-2000);
	ent_create(ship_b1, my.x, ship_function);  // create / place ship at my.x?
	}
	if (bmapno ==2)
	{
	creditsf_add(-3000);	
	ent_create(ship_b2, my.x, ship_function);  // create / place ship at my.x?
	}
	if (bmapno ==3)
	{
	creditsf_add(-500);	
	ent_create(ship_b3, my.x, ship_function);  // create / place ship at my.x?
	}
}

function tile_event() 
{
	if (bmapno ==1 && credits >0) 
	{
	creditsf_add(-2000);
	ent_create(ship_b1, my.x, ship_function);  // create / place ship at my.x?
	}
	if (bmapno ==2 && credits >0)
	{
	creditsf_add(-3000);	
	ent_create(ship_b2, my.x, ship_function);  // create / place ship at my.x?
	}
	if (bmapno ==3 && credits >0)
	{
	creditsf_add(-500);	
	ent_create(ship_b3, my.x, ship_function);  // create / place ship at my.x?
	}
}

function tile_event() {
	if (bmapno ==1) {
		if (creditsf_add(-2000)) { // transaction would be above 0? 
			wait(1);
			ent_create(ship_b1, my.x, ship_function);  // create / place ship at my.x?
			goto(fExit);
		}
	}
	if (bmapno ==2) {
		if (creditsf_add(-3000)) {	
			wait(1);
			ent_create(ship_b2, my.x, ship_function);  // create / place ship at my.x?
			goto(fExit);
		}
	}
	if (bmapno ==3) {
		if (creditsf_add(-500)) {	
			wait(1);
			ent_create(ship_b3, my.x, ship_function);  // create / place ship at my.x?
			goto(fExit);
		}
	}
	fExit:
}