I would lure my friends and family with a bbq and unlimited beer
Hiab would perhaps be cheaper!
Well I'm half tempted to chuck an old mattress on the ground, rope up the beam to a tree or a neighbour's fence, then ask him to drive away fairly quickly!
It's not actually a terrible idea but I would do it to a tree or your own car rather than a neighbour's fence
Find out more about the vehicle? If it's lying flat on a bed with drop sides, just levering it off the side of the truck using a shovel or two, and letting it fall on the old mattress would be fine
Screw a cheap skateboard to it, half way along, flip it over and drag into position
You can also make quite effective manual cranes using 3 bits of wood in an A shape, stand the A next to the truck with the feet of the A a short distance (so it is leaning on the truck), rope over the top of the A, round the middle of the beam, and when you pull the other end of the rope away from the truck, the A will stand up towards vertical (don't pull it all the way over; maybe you can put feet on the A so it stops at the vertical), lifting the beam with little effort from you. The driver can guide the beam by holding one end and you can get it onto the floor just by letting the rope run through your fingers
The more vertical the A starts off at, the less effort required from you but the less distance the beam is lifted (levers are force/distance multipliers; less distance ). For a lever starting off at 75 degrees your pull on the rope is about a quarter the weight of the item. If you go this route you can always practice your technique before the delivery turns up..
I'm also struck that you might be able to find someone else to sell you the beam who will turn up with a hiab.. if it's 20 quid more expensive from builders merchants X than timber yard Y, it'll be worth it if the BM's vehicle is a hiab. Might have to double check though that the BM isn't going to just take your money, place the order with the timber yard and it's the timber yard's truck that turns up